Axial flow gas turbine engines are used to power modern aircraft. These gas turbine engines typically include a compressor section, a combustion section, and a turbine section. A flow path for working medium gases extends axially through the sections of the engine.
As the gases flow along the flow path, the working medium gases are compressed in the compressor section. The working medium gases then flow to the combustion section where they are mixed with fuel. The gases and fuel are burned to add energy to the gases. The gases are expanded through the turbine section to produce useful work to power the compression section and, in case of aircraft engines, to power the aircraft.
Typically, the combustion section includes combustion chambers wherein air compressed by the engine's compressor, is mixed with fuel sprayed into the combustion chambers by fuel nozzles which extend into the combustion chambers. Each combustion chamber includes a bulkhead at the upstream end and a combustion zone axially downstream of the bulkhead. The bulkhead has a plurality of openings to accommodate the extension of fuel nozzles into the combustion chamber. The fuel-air mixture in the combustion zone flows in a whirling flow or vortex pattern as it moves axially downstream further into the combustion chambers.
It will be appreciated that the environment within a gas turbine engine combustion chamber is extremely harsh. The fuel-air mixture burns in the combustion chamber at temperatures as high as 2700.degree. C. (4500.degree. F.) causing extreme thermal gradients and therefore, thermal stresses in the combustion chamber walls. Air is introduced in the combustion chamber to support the fuel combustion process and to cool the combustion chamber in order to relieve thermal stresses. The bulkhead area is one region that requires cooling air because of the heat generated by the introduction and ignition of the fuel-air mixture proximate thereto. Various schemes have been employed to supply cooling air to this area. Typically, air is bled from the compressor to provide the cooling air to the combustion section.
In the prior art, bulkhead cooling air is discharged into the combustion chambers from several bulkhead locations. One portion of the cooling air enters the chamber through standoffs between the bulkhead and the fuel nozzle guides. This portion of cooling air enters the combustion zone and mixes with the fuel-air vortex injected by the fuel nozzles. Another portion of the bulkhead cooling air flows radially outwardly into the combustion chamber proximate to the combustion chamber liners and away from the fuel nozzles. The amount of cooling air entering these prior art combustion chambers through the different locations varies depending on local conditions such as pressures, temperatures and fuel-air mixture. This variability of cooling air entering the combustion process provides for unoptimized and unpredictable combustion.
The introduction of cooling air into the combustion chamber necessarily affects the combustion process or combustion stoichiometry by supplying some of the air (oxygen) to burn the fuel and by guiding the location of flames inherent in the combustion process. For reasons of combustor performance and durability, a primary flame axially downstream of the fuel nozzle is desired. The cooling air, being oxygen rich, may produce undesirable secondary flames at any point after introduction into the fuel rich zone downstream of the fuel nozzles. The local conditions, such as pressures, temperatures, may be able to extinguish these secondary flames. Therefore, the amount of cooling air, the point of its introduction and capability to mix with the fuel-air mixture injected by the fuel injectors, are important in the design of a gas turbine engine combustion chamber.
Due to extreme turbulent and thermodynamic conditions inherent in the combustion chambers, there are localized low pressure areas present near the bulkhead. The localized low pressure areas are prone to the generation of local eddies or swirlings pattern flow of fuel-air mixture introduced by the fuel nozzles. As a result, the localized eddies of the fuel-air mixture proximate to the bulkhead increases the temperature of the bulkhead. The eddies of fuel-air mixture may ignite causing secondary flames which may damage the bulkhead and associated fuel nozzle guides or combustion chamber walls. Even if the eddies do not ignite, they interfere with the axial flow of the fuel-air vortex downstream of the bulkhead. The eddies detract from the creation of useful heat as they trap portions of the fuel-air mixture proximate to the bulkhead depleting the fuel-air vortex of useful energy.
In order to prevent the continuous bathing of the fuel nozzles and bulkhead surfaces with the hot fuel-air mixture eddies, it is important to maintain the conical shape of the fuel-air vortex and to urge the flow of this vortex away from the metal surfaces of the bulkhead and associated structures. Prior art combustion chambers utilize compressor air added directly at the fuel nozzle tip, through dedicated openings to encourage the fuel-air vortex to remain conical as it exits the fuel nozzle to maintain proper fuel-air mixture proportions.
Further, the localized eddies of fuel-air mixture increase the overall temperature of the combustion chamber and thus, may produce unacceptably high levels of Nitrous Oxide (NOx). Efforts to reduce the amount of cooling air are effective in reducing NOx emissions but often cause unacceptable levels of carbon and soot (smoke) formation. The trade between NOx levels and smoke has been a long-standing problem in prior art combustion chambers.